1. Field of the Invention
The invention relates to improved integrated methods of making vinyl acetate monomers from ethane or ethylene using metal oxide catalysts.
2. Description of the Related Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains and are hereby incorporated by reference.
The utilization of lower alkanes (C.sub.1 -C.sub.4) as feed stock to produce value added petrochemicals is an industrially desired process. Lower alkanes are low cost and environmentally acceptable because of their low chemical reactivity. There are only a few commercially available chemical catalytic processes, which utilize lower alkanes as a feed, such as butane to maleic anhydride.
Vinyl acetate monomer (VAM) is a well-known industrial chemical. The production of VAM from ethylene, oxygen and acetic acid using conventional VAM catalysts is known in the art. VAM is typically used as a raw material for vinyl resins such as polyvinyl acetate. VAM was previously primarily manufactured from the vapor phase reaction of ethylene, acetic acid and oxygen with a zinc acetate catalyst.
More recently, VAM has been produced from the vapor-phase reaction of ethylene, acetic acid and oxygen, with a palladium catalyst. For example, VAM may be made from ethylene wherein the first step involves reacting the ethylene to form acetic acid, followed by a second step of reacting a mixture of the acetic acid and ethylene to form vinyl acetate.
Numerous methods are known for the catalytic oxidation of ethylene to acetic acid. See, for example, U.S. Pat. Nos. 3,792,087 and 3,970,697. Similarly, numerous methods are known for the catalytic production of vinyl acetate by reacting ethylene with acetic acid and oxygen in the gaseous phase. See, U.S. Pat. Nos. 3,190,912; 3,637,819; 3,650,896; 4,370,492; 5,185,308; and 4,902,823.
PCT Patent Publication WO 98/05620 describes the production of acetic acid and/or vinyl acetate from ethylene (or ethane) using a first catalyst active for the oxidation of ethylene to acetic acid and/or active for the oxidation of ethane to acetic acid, ethylene and carbon monoxide, and a second catalyst active for the production of vinyl acetate. The patent also describes an additional necessary step for conversion of carbon monoxide to carbon dioxide. This is because carbon monoxide is poisonous to the VAM catalyst.
U.S. Pat. No. 4,188,490 relates to a catalytic oxidation process for the production of mixtures of acetic acid and vinyl acetate comprising the step of contacting a feed mixture containing ethylene, oxygen and water (as steam) with a catalyst composition to provide a mixture of acetic acid and vinyl acetate. The catalyst system comprises a palladium metal on a zinc oxide support treated in the presence of a sulfur modifier. The method requires the subsequent step of fractional distillation to separate the acetic acid from the vinyl acetate. Alternatively, the acetic acid contained in the product mixture is converted in situ to an alkali metal salt such as sodium acetate. The method also requires the step of treating the catalyst with the sulfur modifier by, for example, flowing moist air containing SO.sub.2 over the catalyst at 200.degree. C. for about one hour.
Several methods for producing vinyl acetate from ethylene result in the production of carbon monoxide. The production of carbon monoxide is disadvantageous because it is poisonous to the second stage catalyst. Moreover, carbon monoxide is also a less desirable by-product due to environmental law constraints. In order to avoid this problem, it is necessary to introduce another catalytic reactor for the total oxidation of CO to CO.sub.2. This can add significant costs to the catalytic process.
Accordingly, it would be desirable to provide an improved method for the selective production of vinyl acetate monomer from ethane without the production of carbon monoxide.